Ferrite tuned coaxial magnetron



y 1967 D. c. BUCK FERRITE TUNED COAXIAL MAGNETRON 2 Sheets-Sheet l Filed Dec. 12, 1966 M #RR TN I y// FIG.2.

United States Patent 3,333,148 FERRITE TUNED COAXIAL MAGNETRON Daniel C. Buck, Hanover, Md., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 12, 1966, Ser. No. 601,062 6 Claims. (Cl. 315-39.55)

This invention relates to coaxial magnetrons and more particularly to ferrite tuning means for rapidly varying the frequency of the coaxial magnetrons.

There are many applications where it is desirable to tune a coaxial magnetron rapidly by the application of an electrical signal. It is especially desirable if the tuning can be rapid enough to enable the magnetron to be tuned over a reasonable frequency interval (10% of the center frequency) from pulse to pulse. Since typical pulse repetition rates are about 1 kilohertz, one must be able to tune for instance an X-band magnetron over one gigahertz band in less than one millisecond, in a random manner. Coaxial magnetrons are presently being tuned by various mechanical techniques, all of which employ some sort of movable metal part in the interaction circuit or the external cavity of the tube. Moving metal parts of the order of a few ounces or more cannot be done at several kilohertz rates in a random manner without the application of unreasonable amounts of driving energy. The one exception to this in fast tuning is the spin tuned tube, where the speed is obtained by rapidly spinning the tuner, at the expense of randomness in pulse to pulse frequency selection.

It is therefore an object of the present invention to provide an improved tunable coaxial magnetron.

It is another object to provide a ferrite tuned coaxial magnetron.

It is another object to provide a ferrite tuned coaxial magnetron with minimum spin-wave instabilities.

It is another object to provide a pulsed ferrite tuned coaxial magnetron.

Briefly, the present invention accomplishes the above cited objects by providing a coaxial magnetron utilizing ferrite elements located within the external cavity reso nator and with tuning means associated therewith for controlling the tuning of the coaxial magnetron.

These and other objects and advantages of the present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIGURE 1 is a perspective sectional view of a coaxial magnetron incorporating the teachings of this invention;

FIG. 2 is an elevational view of the coaxial magnetron shown in FIG. 1 illustrating the magnetic circuits;

FIG. 3 is a fragmentary view illustrating a modification of the tuning ring shown in FIG. 1;

FIG. 4 is a sectional view of a coaxial magnetron illustrating another modification of the invention;

FIG. 5 illustrates a circuit for driving the system shown in FIG. 4; and

FIG. 6 is a fragmentary view illustrating a modification of the location of the tuning ring in FIG. 4.

With reference to FIGS. 1 and 2, there is shown a coaxial magnetron embodying the present invention. The magnetron is comprised of a body or shell member 10 which is substantially cup-shaped. The body 10 includes an outer wall member 12 forming the outer wall of an external cavity resonator 30 and a bottom wall member 14 forming the lower wall of the cavity resonator 30. The body member 10 is of a suitable electrically conductive material such as copper. An upper movable member 16 of similar material provides the upper wall of the cavity resonator 30.

Output energy from the magnetron is derived from "ice the external cavity resonator 30 by means of suitable coupling means 18 through the Wall 12. An anode 20 is provided within the body member 10. The anode 20 includes a cylindrical member 23 which defines the inner Wall of the cavity resonator 30. The cylindrical anode portion 23 is secured to the lower plate 14 and the upper movable member 16 is adjacent thereto. The anode 20 includes a plurality of vanes 24 which extend radially inwardly from the cylindrical portion 23.

Centrally disposed and extending through apertures in the end plate 14 is a cathode sleeve 26 which is provided with an electron emissive coating 28 of a suitable material such as barium oxide. This is the basic structure of a coaxial magnetron which includes the inner resonant system formed by the cylindrical anode portion 23 together with the plurality of anode vanes 24. The vanes 24 define a circumferential array of inner, or anode, cavity resonators surrounding the cathode 26. The outer cavity resonator 30 is defined between the outer wall 12, the cylindrical anode portion 23 and the end plates 14 and 16. These two resonant systems are coupled by a circumferential array of uniformly spaced slots 21 through the cylindrical anode portion 23. The inner-resonant system is designed to oscillate in the 11' mode while the outer system is designed to oscillate in the TE mode. The two resonant systems are effectively locked together by means of the coupling slots 21.

The magnetic circuit of the magnetron includes an upper pole piece 31 extending through an aperture provided in the upper plate 16. A cathode pole piece 32 extends through the bottom plate 14. Two substantially horse-shoe magnets 34 as shown in FIG. 2 provide the necessary magnetic energy to the pole pieces 31 and 32 and the unidirectional field within the interaction region defined between the cathode 26 and the anode 20.

In the specific device shown, mechanical tuning is provided by axially moving the upper plate 16 to modify the dimensions of the cavity resonator 30. The annular tuning member 16 defines the upper boundary of the cavity resonator 30. The tuning member 16 is of electrically conductive material. The tuning member 16 is actuated by means of two rod members 46 which extend through apertures 48 in the plate 44. The rods 46 have one end fixed to the annular member 16 and the other end fixed to a cross-bar member 52. The upper pole piece 31 is provided with an extended portion 54 which extends through an aperture in the cross-bar member 52 and into a bore 56 within an actuating rod 60. The rod 60 is attached to the cross-bar member 52 and is slidably mounted in a sleeve member 62 surrounding the rod 60. The sleeve member 62 is secured to a U-shaped magnetic spacer member 64. The rod 60 may be moved within the sleeve 62 and provide movement of the annular member 16 to adjust dimension of the cavity resonator 30. In this manner, mechanical tuning of the magnetron is obtained.

Auxiliary tuning of the coaxial magnetron is accomplished in this specific embodiment by providing an annular ferrite ring 70 of a suitable material such as one of the class of substituuted yttrium iron garnets having saturation magnetization of 400 to 1600 gauss, and having a cross-section dimension of about .050" x .250. The ring 70 is placed on the inner surface of the outer wall 12. The ring 70 may be secured to the wall 10 by suitable means such as heating the wall 12 to 200 C., pressing the ferrite into place and subsequently slow cooling to room temperature. This positions the ferrite ring 70 on the outside portion of the TE cavity mode region. The size of the ferrite member 70 depends on the tuning range desired. The ring 70 is illustrated as rectangular in crosssection. It may be desirable to provide a convex inner surface such that a ring such as shown in FIG. 3 is thicker in the center portion. This convex construction of ring 71 will reduce spin wave instabilities. The microwave magentic field distribution within the tuning ring, is made more uniform by this construction.

An annular electromagnetic member 72 is positioned about the outer surface of the outer Wall 12 and provides a magnetic field as indicated by the arrow 53. The electric field of the TE circular electric mode is indicated by item'74 and its magnetic field is indicatedby the dotted line 76. The tuning D.C. magnetic field indicated by line 53 from the electromagnetic member 72 is provided by suitable current passing through a coil or winding 80 pro-' vided on the ferromagnetic flux guide 82. The flux is directed to the cavity 30 by the guide 82 in such a way that the predominant amount of the 'flux is parallel to 'the TE mode pointing vector indicated by arrow 84 which gives the direction ofthe linearly polarized microwave power flow. With the above device and ferrite dimensions as indicated a tuning range of one percent of the operating frequency of the magnetron has been obtained. A ten percent can be achieved by using a ferrite cross section of about .25 x 25" and a smaller annular cavity.

The ferrite ring 70 can be placed in the end region 14 of the cavity if desiredplt is also possible if desired to provide a vacuum sealed ceramic cylinder within the cavity 30 between the anode wall 23 and the outer .wall 12 so as to permit the ferrite tuning to be accomplished external of the vacuum system. 7

With regard to FIGURE 1, the magnetic field from member 72' is supplied in such a way that the magnetic field on the axis is in no way affected. The magnetic fields are so arranged that the predominant tuningfield in the ferrite is radial whereas its axial component on the axis of symmetry is small. Thus, the magnetizing field has little effect on the unidirectional magnetic field provided by the magnets 34 and has little reaction in the interaction region of the magnetron. It may be'desirable to use magnetic shielding for preventing the unidirectional magnetic field from swamping out the tuning efiect of the tuning field.

It may be also desirable to utilize a bias field above which the tuning field is applied to the electromagnetic member 72. It is found that spin wave instability occurs at critical R.F. powers that vary with DC. magnetic field. The tuning region with bias gives steeper tuning rates When a plane electromagnetic wavetravels through a ferrite with a DC. field normal to the direction of propagation and the alternating field, there results an effective permeability given by the following equation:

-Hill, 1962. The tuning range can be roughly estimated by stating that the relative tuning range is approximately the ratio of the cavity electrical volume with the ferrite included to that with the ferrite volume excluded. This is because for zero tuning field the ferrite looks to the cavity field like a dielectric and when the tuning field is a critical value, the eifective permeability is zero, thus rejecting the cavity fields from it. Since garnets have effec- 4 be achieved with ferrites with dielectric loss tangents of less than .001. The typical figures found for garnets are about .00005.

Referring to FIG. 4, there is illustrated a modified coaxial magnetron configuration. The magnetron consists of a cathode 100 and an anode 92. The anode consists of a cylindrical wall 94 having inwardly extending vanes 96 and slots 98 in the cylindrical wall 94. This structure is similar to that described with respect to FIG. 1. The cathode 100 is centrally located within the anode cavity and pole pieces 101 and 102 are provided to shape the desired unidirectional magnetic field within the anode cavity. An annular cavity resonator 104 surrounds the anode and consists of two end walls 106 and 108, and a slotted cylindrical inner wall 94 and an outer wall 109. An inner ceramic cylindrical wall or sleeve 110 is provided about the anode 92 to permit the coaxial cavity to be separated from the evacuated region defined by the anode 92. The lower cavity end wall 106 is electrically connected through a connector 115 to the anode 92. The upper cavity end wall 108 is secured mechanically to the ceramic sleeve 1.10 and is therefore not in direct electrical contact with the anode 92. Mounted on the upper surface of the end Wall 108 is an annular electromagnetic member 121 including core mamber 117 having a toroidal winding 119. The electromagneticrmember 121 surrounds the anode 92 as illustrated. Anelectrically conductive cap member 112, consisting of a cylindrical portion 114 and an annular portion 116, encloses the electromagnetic member 121. The cylindrical portion 114 is in electrical contact with the member 108 and the annular portion 116 is electrically connected to the anode 92 by an electrically-conductive connector 123. The electromagnetic member 121 forms the primary of a pulse transformer and members 94, 106, 108, 109, 114, 116, 115 and 123 form a secondary winding.

In FIG. 5, a schematic circuit for modulation of the electromagnetic member 121 consisting of a battery 120, a charging diode 122, an SCR device 124 and a suitable transmission line 126 is shown. The secondary of the transformer defined above is designated by the member 128 in FIG. 5. A ferrite ring 130 is provided on the inner surface of the wall 109 and is of similar material as the ferrite ring 'ZOdescribed with respect to FIG. 1.

In the operation of the device illustrated in FIGS. 4 and 5, the SCR device 124 is triggered by a pulse from source 131. The SCR device 124 is triggered into passing some fifty amperes peak current'through the primary winding'119. Peak current in the secondary winding 128 is in the range of 1000 amperes yielding several hundred oersteds tuning field in the ferrite ring 130. Thecurrent located at one end of the cavity and typically contains 50 ment that the tuning magnetic field 53 is tive dielectric constants of about 16-, the ferrite part ofturns of'wire wound toroidally around the tape. wound core. Any suitable method of inducing current into the walls of the cavity could be used.

The relationship of the fields in the cavity is illustrated in FIG. 4. The F 1 magnetic field 76 is axial at the tuning ring 130, the T5 electric field 74 is azimuthal and the tuning magnetic field 53 is azimuthal. The requireperpendicular to the TE magnetic field 76 is met. r a

In FIG. 6, the ferrite ring 130 is located at the inter-.

section of walls 109 and 106. This arrangement reduces the strength of the RF. magentic field in the ferrite ring 130. This is important for high power tubes. In a ferrite,

be as great as 1000 in order to minimize losses. This can there is a critical field for high power saturation which is essentially a characteristic of the material. Above this peak power level, the RE. field will couple to spin waves within the material, appearing to the microwave signal at a loss. One way to raise the peak power level of a ferrite tuned magnetron is to place the ferrite ring 130 in a region of lower R.F. magnetic field. FIG. 6 shows the optimum point for this. The RF. field is reduced in the corner regions, whereas the ferrite ring 130 is still in a position where it can be conveniently mounted, as described before, and conduction cooled to the metal cavity wall.

There is a tradeoif between the peak power capability and tuning range. If the ferrite ring 130 were in zero R.F. magnetic field, there would be no tuning, and infinite peak power capability (as far as spin waves are concerned). In the case of FIG. 6, a reduction in tuning range of about 3 and an increase in peak power level of about 10 was achieved by placing the ferrite ring 130 in the end of the cavity.

Various other modifications may be made within the spirit of the invention.

I claim as my invention:

1. A coaxial magnetron comprising a cathode, a plurality of anode resonators adjacent said cathode and defining an interaction region, magnetic means for establishing a direct current magnetic field within said interaction region, a circular cavity resonator adjacent said anode resonators, said anode resonators and said cavity resonator having a common wall portion with slots therein for coupling selected anode resonators to said cavity resonator, tuning means provided on one of the inner walls of said cavity resonator other than said common wall, said tuning means including a ferrite material, an electromagnetic means exterior of said cavity resonator for modifying the efiective permeability of said ferrite material and thereby modify the frequency of said coaxial magnetron.

2. The coaxial magnetron defined in claim 1 in which said tuning means is a ring member.

3. The coaxial magnetron defined in claim 1 in which a dielectric wall is provided between said common wall portion and said cavity resonator.

4. A coaxial magnetron as defined in claim 2 in which said tuning ring has a curved surface.

5. A coaxial magnetron as defined in claim 1 in which said electromagnetic means includes an annular pulse transformer located coaxially with said cavity resonator and having one turn secondary including the walls of said cavity resonator.

6. A coaxial magnetron as defined in claim 2 in which said ring member is located at the end of said cavity resonator.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner. 

1. A COAXIAL MAGNETRON COMPRISING A CATHODE, A PLURALITY OF ANODE RESONATORS ADJACENT SAID CATHODE AND DEFINING AN INTERACTION REGION, MAGNETIC MEANS FOR ESTABLISHING A DIRECT CURRENT MAGNETIC FIELD WITHIN SAID INTERACTION REGION, A CIRCULAR CAVITY RESONATOR ADJACENT SAID ANODE RESONATORS, SAID ANODE RESONATORS AND SAID CAVITY RESONATOR HAVING A COMMON WALL PORTION WITH SLOTS THEREIN FOR COUPLING SELECTED ANODE RESONATORS TO SAID CAVITY RESONATOR, TUNING MEANS PROVIDED ON ONE OF THE INNER WALLS OF SAID CAVITY RESONATOR OTHER THAN SAID COMMON WALL, SAID TUNING MEANS INCLUDING A FERRITE MATERIAL, AN ELECTROMAGNETIC MEANS EXTERIOR OF SAID CAVITY RESONATOR FOR MODIFYING THE EFFECTIVE PERMEABILITY OF SAID FERRITE MATERIAL AND THEREBY MODIFY THE FREQUENCY OF SAID COAXIAL MAGNETRON. 